PG9.2 and FDW query planning.

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Hello.

I'm in the process of porting our multicorn extension to pg9.2, and
I'd like to take advantage of the GetForeignPaths hook.

The multicorn extension allows the creation of a FDW in python using a
simple API, and I'd like to be able to provide FDW implementors a way
to influence the planner.

Let's say I have an IMAP foreign data wrapper, and I write a query
joining the table on itself using the In-Reply-To and Message-ID
headers, is there anything I can do to avoid fetching all the mails
from the remote server ?

If I could somehow inform the planner that it can look up rows by
message-id, thus avoiding the need to fetch everything from the remote
server. Perhaps "persuading" the planner to use a nested-loop ?

If I understand correctly, I should build one (or more) list of
pathkeys, and add more foreign paths using those pathkeys.

There was a discussion about index on foreign tables back in march.
- From what I understand from this discussion, someone proposed to
locally store information about indexes on the foreign tables, but I
did not find anything on how to build a path "from scratch".

Thank you.

- -- 
Ronan Dunklau

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